I know there are 7 tokens due to a Stage III card but I don't know why my 7th token is a Tassadar hero token

biso said:

They put two because there can exist a rare occasion that does not happen very often and is merely improbable nevertheless possible...

There's a Stage III Event Card that lets you play one of your Stage II Ld Card. So you might get two heroes that way. Finally it's not that much improbable, just need to pick the right card... and keep your first hero alive!